Integrated pyroelectric infrared sensors using PbTiO.sub.3 thin films are well-known in the art. Prior art radiation sensing arrays have been constructed to sense infrared radiation using a PbTiO.sub.3 thin film on a Pt-coated mica, silicon crystal or silicon membrane where the thin film was deposited by RF sputtering. Some prior art methods have employed a linear array sensor having up to 16 elements. Operating parameters of these devices have been characterized and are well known.
PbTiO.sub.3 has been of interest due to its ability to operate at room temperature. Room temperature operational infrared sensors can be used for such applications as remote sensing, biomedical tomography and gas detection. Pyroelectric infrared sensors enjoy some unique advantages over other sensors such as photon sensors. Photon sensors operating in the mid to far infrared region suffer from very low operating temperature requirements which do not provide the advantage of pyroelectric detectors which can be operated at room temperature and operate well throughout the infrared region. Pyroelectric detectors have small wavelength dependence over a wide infrared range and enjoy a fast response time. PbTiO.sub.3 shows excellent pyroelectric characteristics because of its large pyroelectric coefficient and high Curie temperature.
Prior art pyroelectric detectors have been constructed from materials such as PbTiO.sub.3 ceramics and LiTaO.sub.3 single crystals. The prior art employed thin film construction techniques to manufacture planar devices. FIG. 1A shows a PbTiO.sub.3 thin film device constructed with RF sputtering. The device has been fabricated as an infrared sensitive linear array which has a structure of PbTiO.sub.3 thin film on a silicon substrate.
The structures in FIG. 1A have been constructed of a thin film of PbTiO.sub.3 110, a silicon membrane 160, a silicon dioxide layer 150 and an aluminum conductor or Au-black conductor 120. Substrates were created also from a mica sheet 140 with a thickness of 20-50 microns or a silicon single crystal 140. To thin out the silicon membrane, the silicon was preferentially etched into a rectangular thin layer of about 5-20 microns thickness. Platinum/titanium electrodes 130 with 16 elements were formed on a substrate. The PbTiO.sub.3 110 thin film has a thickness of about 2.1 microns which is deposited by RF sputtering. An aluminum or Au-black layer 120 is formed as an infrared absorbing electrode. The array sensor was electrically poled before measuring the infrared response by applying a high electric field to the PbTiO.sub.3 110 thin film at high temperature. This PbTiO.sub.3 pyroelectric infrared sensor is described in more detail in a publication of the faculty of Engineering Science, Osaka University, Japan, entitled "Integrated Pyroelectric Infrared Sensor Using PbTiO.sub.3 Thin Film", Masanori Okuyama, Hiroyuki Seto, Motohiro Kojima, Yasushi Matsui and Yoshihiro Hamakawa, Proceedings of the 14th Conference (1982 International) on Solid State Devices, Tokyo, 1982; Japanese Journal of Applied Physics, Volume 22 (1983) Supplement 22-1, pp. 465-468.
FIG. 2 shows an equivalent circuit 200 of the array sensor of Okuyama, et al. The photodetection element of the array is modeled as a connection of a current source in parallel with a capacitor 210. A pyroelectric current induced under illumination fills the capacitor 210 with charge proportional to the integral of current over time. The charge is then switched through a connecting FET 220 by applying a gate pulse with a sample interval. The circuit 200 is then connected to a current amplifier for sampling. Current sampling effectively resets the sensor which is required by each sampling period.
An alternate integrated pyroelectric infrared sensor is described in an article from the faculty of Engineering Science at Osaka University, Japan, dated Dec. 17, 1984, found in the International Journal of Infrared and Millimeter Waves, Volume 6, No. 1, 1985 entitled "Si Monolithic Integrated Pyroelectric Infrared Sensor Made of PbTiO.sub.3 Thin Film" by Masanori Okuyama, Kohzo Ohtani, Toshi-yukiueda, and Yoshihiro Hamakawa. In this sensor, the PbTiO.sub.3 thin film is sputtered onto a silicon wafer.
Referring to FIG. 1B, a three dimensional drawing of the method of Okuyama, et al. 1985 is shown. In Okuyama, et al. 1985, the PbTiO.sub.3 device is now constructed as an interdigitized sensor built out of silicon cantilevers or silicon bridges. The approach described in the 1985 Okuyama, et al. paper suffers from similar thermal problems of the device in the Okuyama, et al. 1984 paper. Such silicon structures whether solid substrate bridges or cantilevers are also poorly thermally isolated.
Therefore, it is one motivation of this invention to provide a novel PbTiO.sub.3 based pyroelectric sensor that is more sensitive to incoming infrared radiation by providing a means of more effective thermal isolation. The invention advantageously utilizes micromachining techniques. Techniques of dynamic micromechanics on silicon have been well-known in the art. A good review of dynamic micromachining techniques can be found in an article by Curt E. Peterson in the IEEE Transactions on Electronic Devices, Vol. ED-25, No. 10, October 1978, page 1241-1250. Silicon based dynamic micromechanics has been used for a number of applications including oil film projection systems, light valves, thin metal-coated SiO.sub.2 membranes, piezoresistive strain and pressure sensors, deflectable plated-metal cantilever beams, frequency filters, gas chromatography, and hydraulic valves utilizing silicon membranes among others. Micromechanical devices have been constructed of a thin insulated membrane attached to a silicon substrate at one end and suspended over a pit in the silicon. The pit is constructed by silicon etching from under the deposited insulating film using a preferential etching procedure.